Self adjusting automatic load brake

ABSTRACT

A self-adjusting automatic load brake for a hoist is disclosed. In various embodiments, the load brake includes a first shaft defining an annular hollow portion; a second shaft defining an engagement portion, the engagement portion configured for sliding disposition within the annular hollow portion; a first reaction plate coupled to the first shaft; a second reaction plate coupled to the second shaft; and a plurality of friction discs, with at least one of the plurality of friction discs coupled to a cup and disposed between the first reaction plate and the second reaction plate, the annular hollow portion of the first shaft and the engagement portion of the second shaft being sized and configured to accommodate thinning of the plurality of friction discs.

FIELD

The present disclosure relates generally to hoists and, more particularly, to hoists having automatic load brake mechanisms.

BACKGROUND

Hoists used in rescue or related equipment typically wind or unwind a cable in order to raise or lower persons or cargo from a platform. For example, a rescue hoist may be mounted to a frame of an aircraft, such as a helicopter. The rescue hoist may include a cable drum to which a cable is attached. The cable drum rotates in one direction to spool the cable onto the drum and in the other direction to spool the cable off the drum, with one end of the cable attached to the cable drum and the other end, which may include a hook or other device, freely deployed. The hoist typically includes a gear reduction mechanism disposed between a motor and the cable drum to provide a desired rotational speed of the cable drum during operation. The gear reduction mechanism typically includes several shafts arranged to induce large torques or radial loads, thus necessitating robust bearings and other supporting components within the hoist. A load brake may be incorporated into the hoist to control operation of the cable drum during the raising or lowering of loads via the cable.

SUMMARY

A self-adjusting automatic load brake for a hoist is disclosed. In various embodiments, the load brake includes a first shaft defining an annular hollow portion; a second shaft defining an engagement portion, the engagement portion configured for sliding disposition within the annular hollow portion; a first reaction plate coupled to the first shaft; a second reaction plate coupled to the second shaft; and a plurality of friction discs, with at least one of the plurality of friction discs coupled to a cup and disposed between the first reaction plate and the second reaction plate, the annular hollow portion of the first shaft and the engagement portion of the second shaft being sized and configured to accommodate thinning of the plurality of friction discs.

In various embodiments, the cup is coupled to a sprag clutch. In various embodiments, the second shaft is coupled to a ring gear configured to be driven by a motor. In various embodiments, the first shaft is coupled to an output shaft configured to drive a cable drum.

In various embodiments, at least one of the plurality of friction discs includes a first wear surface and the engagement portion is configured for sliding disposition within the annular hollow portion in response to thinning of the first wear surface. In various embodiments, the at least one of the plurality of friction discs includes a second wear surface and the engagement portion is configured for sliding disposition within the annular hollow portion in response to thinning of the first wear surface and the second wear surface.

In various embodiments, the second shaft is held stationary with respect to an axial direction and the first shaft is configured to translate in the axial direction with respect to the second shaft. In various embodiments, the first shaft is coupled to a ball ramp assembly configured to translate the first shaft in the axial direction in response to thinning of the first wear surface and the second wear surface. In various embodiments, the ball ramp assembly is disposed between a plate and a ring gear and the plate is coupled to the first shaft.

In various embodiments, a first pressure plate is operably coupled to a first bias element configured to apply an axial load to the first reaction plate, the second reaction plate and the plurality of friction discs via the first pressure plate. In various embodiments, a second bias element is configured to apply an axial preload to the first reaction plate, the second reaction plate and the plurality of friction discs via the first pressure plate. In various embodiments, a second pressure plate is connected to the first shaft.

A drive train for a hoist having a self-adjusting automatic load brake is disclosed. In various embodiments, the drive train includes a housing; an output shaft rotatably disposed within the housing and configured to rotate a cable drum, the output shaft defining an annular hollow portion and configured to translate axially along a longitudinal axis; a driving shaft rotatably disposed within the housing and defining an engagement portion configured for sliding disposition within the annular hollow portion; a cup rotatably disposed within the housing; a first reaction plate connected to the output shaft; a second reaction plate connected to the driving shaft; and a friction disc connected to the cup and disposed between the first reaction plate and the second reaction plate.

In various embodiments, the driving shaft is coupled to a ring gear configured to be driven by a motor. In various embodiments, the friction disc includes a first wear surface and a second wear surface and the engagement portion is configured for sliding disposition within the annular hollow portion in response to thinning of the first wear surface and the second wear surface. In various embodiments, the driving shaft is held stationary with respect to an axial direction and the output shaft is configured to translate in the axial direction with respect to the driving shaft. In various embodiments, the output shaft is coupled to a ball ramp assembly configured to translate the output shaft in the axial direction in response to thinning of the first wear surface and the second wear surface.

A method of braking a hoist having a self-adjusting automatic load brake is disclosed. In various embodiments, the method includes the steps of: operating the hoist having a first shaft defining an annular hollow portion in axial alignment with a second shaft defining an engagement portion, the engagement portion configured for sliding disposition within the annular hollow portion; and urging the first shaft toward the second shaft, such that the engagement portion is slid within the annular hollow portion upon thinning of a plurality of friction discs, with at least one of the plurality of friction discs coupled to a cup and disposed between a first reaction plate coupled to the first shaft and a second reaction plate coupled to the second shaft, the annular hollow portion of the first shaft and the engagement portion of the second shaft being sized and configured to accommodate thinning of the plurality of friction discs.

In various embodiments, the cup is coupled to a sprag clutch, the second shaft is coupled to a ring gear configured to be driven by a motor and the first shaft is coupled to an output shaft configured to drive a cable drum. In various embodiments, the second shaft is held stationary with respect to an axial direction and the first shaft is configured to translate in the axial direction with respect to the second shaft.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter of the present disclosure is particularly pointed out and distinctly claimed in the concluding portion of the specification. A more complete understanding of the present disclosure, however, may best be obtained by referring to the following detailed description and claims in connection with the following drawings. While the drawings illustrate various embodiments employing the principles described herein, the drawings do not limit the scope of the claims.

FIG. 1A is an elevation view of an aircraft and a hoist, in accordance with various embodiments;

FIG. 1B is a cross-sectional view of a hoist, in accordance with various embodiments;

FIG. 2 is a cross sectional view of a drive train of a hoist, in accordance with various embodiments;

FIGS. 3A and 3B are cross sectional views of a load brake mechanism, in accordance with various embodiments; and

FIGS. 3C and 3D are cross sectional views of a load brake mechanism, in accordance with various embodiments.

DETAILED DESCRIPTION

The following detailed description of various embodiments herein makes reference to the accompanying drawings, which show various embodiments by way of illustration. While these various embodiments are described in sufficient detail to enable those skilled in the art to practice the disclosure, it should be understood that other embodiments may be realized and that changes may be made without departing from the scope of the disclosure. Thus, the detailed description herein is presented for purposes of illustration only and not of limitation. Furthermore, any reference to singular includes plural embodiments, and any reference to more than one component or step may include a singular embodiment or step. Also, any reference to attached, fixed, connected, or the like may include permanent, removable, temporary, partial, full or any other possible attachment option. Additionally, any reference to without contact (or similar phrases) may also include reduced contact or minimal contact. It should also be understood that unless specifically stated otherwise, references to “a,” “an” or “the” may include one or more than one and that reference to an item in the singular may also include the item in the plural. Further, all ranges may include upper and lower values and all ranges and ratio limits disclosed herein may be combined.

Referring now to the drawings, FIG. 1A provides a perspective view of an aircraft 10 and a rescue hoist 100 mounted to the aircraft 10, while FIG. 1B provides a cross-sectional view of the rescue hoist 100. The rescue hoist 100 is mounted to the aircraft 10 by a support frame 14 and a cable 16 extends from the rescue hoist 100. In various embodiments, the rescue hoist 100 includes a frame 102, a motor 104, a drive train 106, a cable drum 108 and a level wind mechanism 110. The cable drum 108 includes a first flange 112, a second flange 114 and a barrel 116. The barrel 116 extends between and connects the first flange 112 and the second flange 114. The level wind mechanism 110 includes a level wind gear 118 and a screw 134. The drive train 106 includes a housing 120. The housing 120 includes a first end 122 and a second end 124, with the second end 124 including a mounting flange 126 (or a plurality thereof). The mounting flange 126 may include a fastener opening 128 and an alignment opening 130. The rescue hoist 100 is mounted to the aircraft 10 by the support frame 14. The cable 16 extends from the rescue hoist 100 and is configured to raise and lower objects to and from the aircraft 10. A bearing system 132, such as, for example, a linear bearing, is rotatably mounted to the frame 102. The motor 104 is connected to the frame 102 and is configured to operate the drive train 106, which is configured to transmit rotational power from the motor 104 to the cable drum 108. The level wind mechanism 110 extends through cable drum 108 and is configured to wrap the cable 16 around the barrel 116 between the first flange 112 and the second flange 114. The level wind mechanism 110 is coupled to the motor 104 and configured to translate the cable drum 108 back and forth along a direction defined by a longitudinal axis A via the screw 134.

Referring now to FIG. 2, a cross-sectional view of a drive train 206 used to power a hoist is provided. The drive train 206 is similar to the drive train 106 used to power the rescue hoist 100, described above with reference to FIG. 1B. In various embodiments, the drive train 106 includes a housing 220, which includes a first end 240, a second end 242 and one or more drive slots 246, through which one or more planetary gears 248 may protrude to drive a cable drum, such as, for example, the cable drum 108 described above with reference to FIG. 1B. In various embodiments, the drive train 206 may include several stages and shafts, such as, for example, a first stage 250, a second stage 252, a third stage 254, a first transmission shaft 256 and a second transmission shaft 258. The first stage 250 includes a first epicyclic gear system 260 and a load brake 262. The first epicyclic gear system 260 includes a first set of planetary gears housed within a first ring gear 264. The second stage 252 includes a second epicyclic gear system 266 and an overload clutch 268. The second epicyclic gear system 266 includes a second set of planetary gears housed within a second ring gear 270. The third stage 254 includes a third set of planetary gears 272 (e.g., the one or more planetary gears 248) configured to drive the cable drum. The load brake 262 includes a brake disc pack 274 and an output shaft 276. The overload clutch 268 includes a clutch input 278 connected to the second epicyclic gear system 266. In various embodiments, the overload clutch 268 is configured to slip when a load on a cable, such as, for example, the cable 16 described above with reference to FIGS. 1A and 1B, reaches a set point that is greater than a rated load of the hoist, thereby decoupling the second transmission shaft 258 from the second stage 252, the first transmission shaft 256, the load brake 262, the first stage 250 and the motor 204. As such, the excess load on the cable is prevented from being transmitted to the motor 204 by overload clutch 268, thereby providing overload protection.

In operation, a motor shaft 205 of a motor 204 is configured to mesh with the first epicyclic gear system 260 to drive the first ring gear 264. The first ring gear 264 is connected to and, subsequently, drives the load brake 262. In various embodiments, the load brake 262 is configured to create a proportional clamping force across the brake disc pack 274 in response to tension created by a load on the cable. The load brake 262 prevents slippage of the cable through the proportional clamping of the brake disc pack 274 and thereby facilitates a smooth lowering motion for the cable. The load brake 262 thus assists in controlling the speed at which the cable is lowered by the drive train 206. The load brake 262 also resists movement of the cable when the motor 204 is not activated. In various embodiments, the first stage 250 provides a speed reduction between the motor 204 and the second stage 252 and, as such, the load brake 262 rotates one stage slower than motor 204. The output shaft 276 of the load brake 262 is connected to the first transmission shaft 256 which, in turn, provides power to the second stage 252 and, subsequently, to the third stage 254 via the second transmission shaft 258, thereby driving the cable drum via the third set of planetary gears 272 (or the one or more planetary gears 248).

Referring now to FIGS. 3A, 3B, 3C and 3D, details of a load brake 300, similar to the load brake 262 described above with reference to FIG. 2, are provided. In various embodiments, the load brake 300 includes a brake disc pack 302, comprising a plurality of reaction plates 304 and a plurality of friction discs 306. In various embodiments, each of the plurality of friction discs 306 (e.g., a first friction disc 308) includes a radially outward end 310 that is splined to a cup 312. In various embodiments, the cup 312 extends annularly about the brake disc pack 302 and may be coupled to a sprag clutch 314 configured for one-way rotation. Each of the plurality of friction discs 306 (e.g., the first friction disc 308) includes a first wear surface 316 disposed on one side of the friction disc and a second wear surface 318 disposed on the opposite side of the friction disc. A one of the plurality of reaction plates 304 (e.g., a first reaction plate 320) is disposed between adjacent pairs of the plurality of friction discs 306. Each one of the plurality of reaction plates 304 (e.g., the first reaction plate 320) includes a radially inner end 322 that is splined to one of a driving shaft 324 or a driven shaft 326, either of which may, in various embodiments, be referred to as a first shaft or a second shaft. As illustrated in FIG. 3A, a first subset of the plurality of reaction plates 304 (e.g., the first reaction plate 320) may be splined to the driven shaft 326, while a second subset of the plurality of reaction plates 304 (e.g., a second reaction plate 309) may be splined to the driving shaft 324. In various embodiments, the driving shaft 324 may comprise or be coupled to a first ring gear 364, such as, for example, the first ring gear 264 described above with reference to FIG. 2, while the driven shaft 326 may comprise or be coupled to an output shaft 376, such as, for example, the output shaft 276, also described above with reference to FIG. 2.

In various embodiments, a first bias element 330 is configured to provide an axial load against a first pressure plate 332, which, in turn, provides an axial load within the brake disc pack 302. The first bias element 330 may comprise a spring, such as, for example, a Belleville spring 334. In various embodiments, a second bias element 336 is configured to provide a preload (or an axial preload) within the brake disc pack 302 via the first pressure plate 332. A ball ramp assembly 340 may operatively couples the first ring gear 364 to the output shaft 376 via a plate 338 and be configured to adjust the axial load that exists within the brake disc pack 302. In various embodiments, the ball ramp assembly 340 comprises a ball 342 configured to slide or roll on a ramp 344 disposed on an interior surface of the first ring gear 364. The ball 342 slides or rolls on the ramp 344 when there is relative rotary motion between the first ring gear 364 and the plate 338, which drives the plate 338 in an axial direction away from the brake disc pack 302. The plate 338 is connected to the output shaft 376, such that the plate 338 and the output shaft 376 are configured to rotate about and translate along a longitudinal axis A as a single unit. In various embodiments, the plate 338 and the output shaft 376 are connected to one another by a center bolt 346.

During operation, a motor (e.g., the motor 204 described above with reference to FIG. 2) rotates the first ring gear 364 in either a reel-in direction or a reel-out direction. The cable may be loaded or unloaded in either direction, which changes the operation of the hoist. For example, when operating in the reel-in direction, with no load on the cable (i.e., with no back-driving torque applied to the output shaft 376), the ball ramp assembly 340 assumes a relieved configuration, whereby the plate 338 is disposed at a first axial position 350 with respect to the longitudinal axis A. In this configuration, the cup 312, the brake disc pack 302, the plate 338 and the output shaft 376 all rotate as a unit. When operating in the reel-out direction, with no load on the cable, the cup 312 is grounded by the sprag clutch 314, thereby locking the plurality of friction discs 306 in place. In this mode of operation, the motor provides sufficient torque to the first ring gear 364 to slip the plurality of reaction plates 304 with respect to the plurality of friction discs 306, thereby driving the cable off the cable drum.

Conversely, when operating in the reel-in direction or the reel-out direction, with a load on the cable, a back-driving torque is applied to the output shaft 376, causing the shaft and the plate 338 to rotate relative to the first ring gear 364. The relative rotation between the plate 338 and the first ring gear 364 causes the ball 342 to slide or roll on the ramp 344 disposed on an interior surface of the first ring gear 364, thereby affecting axial translation of the plate 338, together with the output shaft 376, to a second axial position 352 (see, e.g., FIG. 3C) with respect to the longitudinal axis A. Translation of the output shaft 376 causes a second pressure plate 333 to translate toward the first pressure plate 332, thereby clamping together the plurality of reaction plates 304 and the plurality of friction discs 306 and locking the brake disc pack 302. The locking of the brake disc pack 302 provides the automatic braking feature of the hoist, locking the cable drum from rotating. The greater the load on the cable, the greater the axial load applied to the brake disc pack 302. In this mode of operation, the motor is required to overcome the torque created by the brake disc pack 302 assuming a locked configuration to either reel in or reel out the cable.

Still referring to FIGS. 3A-3D, over time, the wear surfaces of the brake disc pack 302, e.g., the first wear surface 316 and the second wear surface 318 of the first friction disc 308, will experience thinning due to frictional wear. The combined thinning of the wear surfaces of the plurality of friction discs 306 will cause a first length L₁ of the brake disc pack 302 (see, e.g., FIG. 3A) to become reduced by a delta length ΔL to a second length L₂ of the brake disc pack 302 (see, e.g., FIG. 3C), where ΔL=L₁−L₂. As described below, the load brake 300 is configured to absorb the delta length ΔL, such that the locking of the brake disc pack 302 is not adversely affected by the thinning of the wear surfaces. In various embodiments, for example, as the wear surfaces experience thinning, an engagement portion 370 of the driving shaft 324 is configured to translate an axial distance ΔX, from a first axial position X₁ to a second axial position X₂, where ΔX=X₁−X₂=ΔL.

To accommodate the thinning wear surfaces, the engagement portion 370 of the driving shaft 324 is configured to slide in an axial direction into an annular hollow portion 372 defined by the driven shaft 326. In various embodiments, the annular hollow portion 372 extends an axial length 380 into the driving shaft 324 of at least the combined thickness of the wear surfaces of the plurality of friction discs 306. For example, as indicated in FIGS. 3B and 3D, the axial length 380 of the annular hollow portion 372 is at least ΔX. Similarly, in various embodiments, the annular hollow portion 372 extends a radial length 382 in the radial direction R. The radial length 382 is configured to accommodate a combined radial thickness of the driving shaft 324, or at least the portion thereof configured to spline the one or more of the plurality of reaction plates 304 to the driving shaft 324. Sized as described above, the annular hollow portion 372 may be configured to receive therein a portion of the driving shaft 324 to accommodate for the thinning of the wear surfaces on the plurality of friction discs 306.

Benefits, other advantages, and solutions to problems have been described herein with regard to specific embodiments. Furthermore, the connecting lines shown in the various figures contained herein are intended to represent exemplary functional relationships and/or physical couplings between the various elements. It should be noted that many alternative or additional functional relationships or physical connections may be present in a practical system. However, the benefits, advantages, solutions to problems, and any elements that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as critical, required, or essential features or elements of the disclosure. The scope of the disclosure is accordingly to be limited by nothing other than the appended claims, in which reference to an element in the singular is not intended to mean “one and only one” unless explicitly so stated, but rather “one or more.” Moreover, where a phrase similar to “at least one of A, B, or C” is used in the claims, it is intended that the phrase be interpreted to mean that A alone may be present in an embodiment, B alone may be present in an embodiment, C alone may be present in an embodiment, or that any combination of the elements A, B and C may be present in a single embodiment; for example, A and B, A and C, B and C, or A and B and C. Different cross-hatching is used throughout the figures to denote different parts but not necessarily to denote the same or different materials.

Systems, methods and apparatus are provided herein. In the detailed description herein, references to “one embodiment,” “an embodiment,” “various embodiments,” etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. After reading the description, it will be apparent to one skilled in the relevant art(s) how to implement the disclosure in alternative embodiments.

Furthermore, no element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims. No claim element herein is to be construed under the provisions of 35 U.S.C. 112(f) unless the element is expressly recited using the phrase “means for.” As used herein, the terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus.

Finally, it should be understood that any of the above described concepts can be used alone or in combination with any or all of the other above described concepts. Although various embodiments have been disclosed and described, one of ordinary skill in this art would recognize that certain modifications would come within the scope of this disclosure. Accordingly, the description is not intended to be exhaustive or to limit the principles described or illustrated herein to any precise form. Many modifications and variations are possible in light of the above teaching. 

What is claimed:
 1. A self-adjusting automatic load brake for a hoist, comprising: a first shaft defining an annular hollow portion; a second shaft defining an engagement portion, the engagement portion configured for sliding disposition within the annular hollow portion; a first reaction plate coupled to the first shaft; a second reaction plate coupled to the second shaft; and a plurality of friction discs, with at least one of the plurality of friction discs coupled to a cup and disposed between the first reaction plate and the second reaction plate, wherein the annular hollow portion of the first shaft and the engagement portion of the second shaft are sized and configured to accommodate thinning of the plurality of friction discs.
 2. The load brake of claim 1, wherein the cup is coupled to a sprag clutch.
 3. The load brake of claim 2, wherein the second shaft is coupled to a ring gear configured to be driven by a motor.
 4. The load brake of claim 3, wherein the first shaft is coupled to an output shaft configured to drive a cable drum.
 5. The load brake of claim 1, wherein at least one of the plurality of friction discs includes a first wear surface and the engagement portion is configured for sliding disposition within the annular hollow portion in response to thinning of the first wear surface.
 6. The load brake of claim 5, wherein the at least one of the plurality of friction discs includes a second wear surface and the engagement portion is configured for sliding disposition within the annular hollow portion in response to thinning of the first wear surface and the second wear surface.
 7. The load brake of claim 6, wherein the second shaft is held stationary with respect to an axial direction and the first shaft is configured to translate in the axial direction with respect to the second shaft.
 8. The load brake of claim 7, wherein the first shaft is coupled to a ball ramp assembly configured to translate the first shaft in the axial direction in response to thinning of the first wear surface and the second wear surface.
 9. The load brake of claim 8, wherein the ball ramp assembly is disposed between a plate and a ring gear and the plate is coupled to the first shaft.
 10. The load brake of claim 9, further comprising a first pressure plate operably coupled to a first bias element configured to apply an axial load to the first reaction plate, the second reaction plate and the plurality of friction discs via the first pressure plate.
 11. The load brake of claim 10, further comprising a second bias element configured to apply an axial preload to the first reaction plate, the second reaction plate and the plurality of friction discs via the first pressure plate.
 12. The load brake of claim 11, further comprising a second pressure plate connected to the first shaft.
 13. A drive train for a hoist having a self-adjusting automatic load brake, comprising: a housing; an output shaft rotatably disposed within the housing and configured to rotate a cable drum, the output shaft defining an annular hollow portion and configured to translate axially along a longitudinal axis; a driving shaft rotatably disposed within the housing and defining an engagement portion configured for sliding disposition within the annular hollow portion; a cup rotatably disposed within the housing; a first reaction plate connected to the output shaft; a second reaction plate connected to the driving shaft; and a friction disc connected to the cup and disposed between the first reaction plate and the second reaction plate.
 14. The drive train of claim 13, wherein the driving shaft is coupled to a ring gear configured to be driven by a motor.
 15. The drive train of claim 14, wherein the friction disc includes a first wear surface and a second wear surface and the engagement portion is configured for sliding disposition within the annular hollow portion in response to thinning of the first wear surface and the second wear surface.
 16. The drive train of claim 15, wherein the driving shaft is held stationary with respect to an axial direction and the output shaft is configured to translate in the axial direction with respect to the driving shaft.
 17. The drive train of claim 16, wherein the output shaft is coupled to a ball ramp assembly configured to translate the output shaft in the axial direction in response to thinning of the first wear surface and the second wear surface.
 18. A method of braking a hoist having a self-adjusting automatic load brake, comprising: operating the hoist having a first shaft defining an annular hollow portion in axial alignment with a second shaft defining an engagement portion, the engagement portion configured for sliding disposition within the annular hollow portion; and urging the first shaft toward the second shaft, such that the engagement portion is slid within the annular hollow portion upon thinning of a plurality of friction discs, with at least one of the plurality of friction discs coupled to a cup and disposed between a first reaction plate coupled to the first shaft and a second reaction plate coupled to the second shaft, wherein the annular hollow portion of the first shaft and the engagement portion of the second shaft are sized and configured to accommodate thinning of the plurality of friction discs.
 19. The method of claim 18, wherein the cup is coupled to a sprag clutch, the second shaft is coupled to a ring gear configured to be driven by a motor and the first shaft is coupled to an output shaft configured to drive a cable drum.
 20. The method of claim 19, wherein the second shaft is held stationary with respect to an axial direction and the first shaft is configured to translate in the axial direction with respect to the second shaft. 